1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and a thin film transistor substrate and, more particularly, a VA (Vertically aligned) mode liquid crystal display device and a thin film transistor substrate.
2. Description of the Prior art
The liquid crystal display device is employed in various electronic devices, e.g., is employed as not only the display of the mobile computer, but also the display of the desk-top computer, the display of the television, the projector, the personal digital assistant (PDA), etc.
The normal TN (Twisted Nematic) mode liquid crystal display device has such a structure that the liquid crystal is sealed between two transparent substrates. Out of two surfaces of these transparent substrates opposing to each other, the common electrode, the color filter, the alignment film, etc. are formed on one surface side, and the thin film transistor (TFT), the pixel electrodes, the alignment film, etc. are formed on the other surface side. Also, polarizing plates are stuck on the opposing surfaces and the opposite side surfaces of the transparent substrates respectively. These two polarizing plates are arranged such that, for example, their polarization axes can intersect perpendicularly to each other. In this case, two polarizing plates give the light display (white display) to transmit the light in the condition that the voltage is not applied between the pixel electrode and the common electrode, while they give the dark display (black display) to cut off the light in the condition that the voltage is applied. In contrast, if the polarization axes of two polarizing plates are arranged in parallel with each other, two polarizing plates give the dark display in the condition that the voltage is not applied between the pixel electrode and the common electrode, while they give the light display in the condition that the voltage is applied. In the following description, the substrate on which TFT and the pixel electrodes are formed is called the TFT substrate, while the substrate on which the color filters and the common electrode are formed is called the opposing substrate.
The TN mode liquid crystal display device has such drawbacks that the viewing angle is narrow and the resolution is not sufficient.
FIGS. 1A to 1C are views showing these drawbacks. FIG. 1A shows the state to display the white by not applying the voltage between two electrodes 101, 102, FIG. 1B shows the state to display the half tone (gray) by applying the intermediate voltage V1 between two electrodes 101, 102, and FIG. 1C shows the state to display the black by applying the predetermined voltage V2 between two electrodes 101, 102.
In FIGS. 1A to 1C, alignment films 103, 104 are formed on the opposing surfaces of two electrodes 101, 102 to differentiate their alignment directions by 90° (degrees) respectively. Also, although not shown, the polarizing plates are arranged on respective outsides of two electrodes 101, 102 in the condition that their linearly polarized directions are twisted mutually by 90 degrees. In this case, actually liquid crystal molecules L shown in FIGS. 1A to 1C are twisted in compliance with the alignment direction of the alignment films 103, 104, but they are illustrated herein not to take account of the twist, for the convenience of explanation.
Meanwhile, as shown in FIG. 1A, in the condition that the voltage is not applied, the liquid crystal molecules L are aligned in the same direction to have a very small tilt angle (about 1 degree to 5 degrees). In this state, the display looks like almost white from all directions.
Also, as shown in FIG. 1C, in the condition that the voltage V2 is applied, the liquid crystal molecules L are aligned in the perpendicular direction to the alignment films 103, 104 except the neighborhood of their surfaces. Since the incident linearly polarized light is intercepted by the plate, the display looks like the black from the outside. At this time, since the light irradiated obliquely into one electrode 101 passes obliquely to the direction of the liquid crystal molecules L aligned in the vertical direction to thus twist its polarization direction to some extent, the display looks like not the perfect black but the half tone (gray) from the outside.
In addition, as shown in FIG. 1B, in the condition that the intermediate voltage V1 lower than the state in FIG. 1C is applied, the liquid crystal molecules L positioned in vicinity of the alignment films 103, 104 are also aligned in the horizontal direction, but the liquid crystal molecules L rise obliquely in the middle area of the cell. Therefore, the double refraction (birefringence) property of the liquid crystal is lost in some degree to lower the transmittance and thus the half tone (gray) display appears. However, this is true of only the light L1 that is irradiated vertically to the liquid crystal panel. The light that is irradiated obliquely to the surface of one electrode 101 exhibits different behaviors when the display is viewed from the left and right directions in FIG. 1B.
In other words, in FIG. 1B, the direction of the liquid crystal molecules L becomes parallel with the light L2 that is directed from the lower right to the upper left. Therefore, since the liquid crystal L seldom exhibits the double refraction effect, the display looks like the black when it is viewed from the left side. On the contrary, the direction of the liquid crystal molecules L becomes perpendicular to the light L3 that is directed from the lower left to the upper right. Therefore, since the liquid crystal L exhibits greatly the double refraction effect to the incident light to twist the incident light, the display looks like a color close to the white. That is, the display intensity is changed according to the viewing angle, and this aspect is the biggest drawback of the TN mode liquid crystal display device.
For this reason, as the mode that can improve the viewing angle characteristic without reduction of the response speed, the VA (Vertically Aligned) mode using the vertical alignment films has been proposed.
FIGS. 2A to 2C are views showing the VA mode. The VA mode uses the negative type liquid crystal material and the vertical alignment films in combination.
First, as shown in FIG. 2A, when the voltage is not applied, the liquid crystal molecules L are aligned in the vertical direction to provide the black display. In the VA mode, the vertically aligning process is applied to the alignment films 103, 104.
Also, as shown in FIG. 2C, when the predetermined voltage V2 is applied between two electrodes 101, 102, the liquid crystal molecules L are aligned in the horizontal direction to provide the while display. The VA mode has the high display contrast, the quick response speed, and the visual characteristic in the white display and the black display rather than the TN mode.
In addition, as shown in FIG. 2B, when the predetermined voltage V1 smaller than that in the white display is applied between two electrodes 101, 102, the liquid crystal molecules L are aligned in the oblique direction. In this case, the light that is perpendicular to the surface of the electrode 101 is displayed as the half tone on the display panel. However, in FIG. 2B, the liquid crystal molecules L are parallel with the light L2 directed from the lower right to the upper left. Accordingly, since the liquid crystal molecules L seldom exhibits the double refraction effect, the display looks like the black if it is viewed from the left side. In contrast, the liquid crystal molecules L are vertical to the light L3 directed from the lower left to the upper right. Accordingly, since the liquid crystal molecules L exhibits greatly the double refraction effect to the incident light to twist the incident light, the display that is close to the white is given.
In this manner, since the liquid crystal molecules positioned in the neighborhood of the alignment films become substantially vertical when the voltage is not applied, the VA mode has the especially high contrast and also is excellent in the viewing angle characteristic rather than the TN mode. However, the VA mode has the problem similar to the TN mode, i.e., when the half tone display is performed in the VA mode, the display intensity is changed if the viewing angle is changed. Thus, the VA mode is still not enough in the aspect of the viewing angle characteristic.
In Patent Application Hei 10-185836, the applicant of this application discloses the configuration in which vertical alignment in the prior art is used, the liquid crystal material having the negative dielectric anisotropy, so-called negative type liquid crystal, is sealed between the electrodes, and the domain defining means for defining the liquid crystal molecules to differentiate their tilt directions in a plurality of regions in one pixel when the voltage is not applied is provided.
FIGS. 3A to 3C are views showing the visual characteristic improving principle by using alignment division. In this case, the structure is employed in which the slit S is formed in one pixel electrode 111 on the first substrate side as the domain defining means and the projection P is provided in one pixel on the electrode 112 on the second substrate side.
As shown in FIG. 3A, when the voltage is not applied, the liquid crystal molecules are aligned perpendicularly to the substrate surface. Also, as shown in FIG. 3C, when the predetermined voltage V2 is applied between the opposing electrodes 111, 112, the liquid crystal molecules are aligned in parallel with the substrate surface to provide the white display.
In addition, as shown in FIG. 3B, when the intermediate voltage V1 is applied between the opposing electrodes 111, 112, the electric field that is oblique to the substrate surface is generated due to the slit (electrode edge portion) S. Also, the liquid crystal molecules L in the neighborhood of the surface of the projection P are slightly tilted from the state when no voltage is applied. The tilt directions of the liquid crystal molecules L are decided by the influence of inclined surfaces of the projection P and the oblique electric field. Thus, the alignment directions of the liquid crystal molecules 113 are divided in the middle of the projection P and in the middle of the slit portion ills respectively.
At this time, since the liquid crystal molecules L are slightly tilted, for example, the light L1 that is transmitted from the bottom of the substrate to the top is affected slightly by the double refraction to suppress the transmission. Thus, the half tone display of gray can be obtained. The light L2 transmitted from the lower right to the upper left is hard to transmit in the area in which the liquid crystal molecules L are tilted to the left direction, but such light L2 is very easy to transmit in the area in which the liquid crystal molecules L are tilted to the right direction. Thus, the half tone display of gray can be obtained as the average. In addition, the light L3 transmitted from the lower left to the upper right exhibits the gray display based on the similar principle. As a result, the uniform half tone display can be obtained in all directions in one pixel.
Therefore, in FIG. 3B, the good display that has the small viewing angle dependency can be obtained in all the black, half tone, and white display states.
In FIGS. 3A to 3C, the slit S is formed in one pixel electrode 111 on the first substrate side as the domain defining means, and the projection P is provided in one pixel on the electrode 112 on the second substrate side. But such structure may be accomplished by other means. Such new VA mode is referred to as the MVA (Multi-domain Vertical Alignment) mode in the following.
FIGS. 4A to 4C are views showing examples for implementing the domain defining means.
FIG. 4A shows an example in which the domain defining means is implemented only by using the electrode shapes, FIG. 4B shows an example in which shapes of the substrate surfaces are designed, and FIG. 4C shows an example in which shapes of the electrodes and the substrate surfaces are designed. Although the alignments shown in FIGS. 3A to 3C can be obtained in all these examples, respective structures are slightly different.
Next, the case where projections are provided on the opposing surfaces of two substrates, as shown in FIG. 4B, will be explained as an example hereunder.
In FIG. 4B, projections P1, P2 for dividing the alignment directions alternatively are formed on the electrodes 111, 112 on the opposing surfaces of two substrates, and also vertical alignment films 113, 114 are provided on the inner surfaces of them. The vertical aligning process is applied to the vertical alignment films. The liquid crystal injected between two substrates is the negative type one. When no voltage is applied, the liquid crystal molecules L are aligned perpendicularly to the substrate surface on the vertical alignment films. Since the liquid crystal molecules L tend to be aligned perpendicularly to the inclined surfaces of the projections P1, P2, such liquid crystal molecules L on the projections P1, P2 are also tilted. However, since the liquid crystal molecules L are aligned almost perpendicularly to the substrate surface in most areas except for the projections P1, P2 when no voltage is applied, the good black display can be obtained, as shown in FIG. 3A.
When the voltage is applied, the liquid crystal molecules L are parallel with the substrate (the electric field is perpendicular to the substrate) in areas in which the projections P1, P2 are not provided, but such liquid crystal molecules L are tilted in vicinity of the projections P1, P2. In other words, when the voltage is applied, the liquid crystal molecules L are tilted in response to the intensity of the electric field but the electric field is directed perpendicularly to the substrate. Therefore, unless the tilt direction of the liquid crystal molecules L is defined by the rubbing, the liquid crystal molecules L may take all directions of 360 degrees as the tilted azimuth to the electric field. Since the electric field is inclined in the direction parallel with the inclined surfaces of the projections P1, P2 on the projections P1, P2, the liquid crystal molecules L are tilted in the direction perpendicular to the electric field when the voltage is applied. This direction coincides with the original direction inclined by the projections P1, P2, and thus the liquid crystal molecules L are aligned more stably. In this manner, the projections P1, P2 can provide the stable alignment by both effects of their inclination and the electric field on the inclined surface. In addition, if the large voltage is applied, the liquid crystal molecules L are aligned in almost parallel with the substrate.
As described above, the projections P1, P2 can perform a role of the trigger that decides the alignment azimuth of the liquid crystal molecules L when the voltage is applied.
In FIG. 4A, slits S1, S2 are provided on both or either of electrodes 111, 112. The vertical aligning process is applied to the alignment films 113, 114, and the negative type liquid crystal is sealed between the substrates. The liquid crystal molecules L are aligned perpendicularly to the substrate surface when no voltage is applied, whereas the electric field is generated at the slits (electrode edge portions) S1, S2 in the oblique direction to the substrate surface when the voltage is applied. The tilt directions of the liquid crystal molecules L are decided by the influence of this oblique electric field, and thus the alignment directions of the liquid crystal molecules are divided in the right and left directions, as shown in FIG. 4A.
FIG. 4C shows an example in which the modes in FIG. 4A and FIG. 4B are combined together. The slits S are formed in one electrode 111 while the projections are provided on the other electrode 112. Though examples for implementing three domain defining means are illustrated as above, various variations may be adopted.
FIG. 5 is a plan view showing positional relationships among bus lines, projections, pixels, and electrodes in the liquid crystal display panel in which the alignment of the liquid crystal molecules are divided into four directions. FIG. 6 is a sectional view taking along a I—I line in FIG. 5.
In FIG. 5 and FIG. 6, a plurality of gate bus lines 122 extending in the X direction (the lateral direction in FIG. 5 and FIG. 6) are formed on the TFT substrate 121 at a distance along the Y direction (the longitudinal direction in FIG. 5 and FIG. 6). Also, capacitive bus lines 123 extending in the X direction are formed between the gate bus lines 122. Auxiliary capacitive branch lines 123a that have a length not to touch the gate bus lines 122 are formed from the capacitive bus lines 123 in the Y direction so as to oppose to a part of drain bus lines (also called data bus lines), described later.
The gate bus lines 122 and the capacitive bus lines 123 are covered with a first insulating film 124. Then, a plurality of drain bus lines 125 extending in the Y direction are formed in the X direction on the first insulating film 124 at a distance. The TFTs 126 are formed to correspond to crossing portions between the gate bus lines 122 and the drain bus lines 125. The TFT 126 has a semiconductor layer 126a formed on the gate bus line 122 via the first insulating film 124, a drain electrode 126d formed on the semiconductor layer 126a, and a source electrode 126s formed on the semiconductor layer 126a. The drain electrode 126d is connected to the neighboring drain bus lines 125. The drain bus lines 125 and the TFTs 126 are covered with a second insulating film 127.
A pixel electrode 128 made of ITO (indium-tin oxide) is formed on the second insulating film 127 and in the area surrounded by two drain bus lines 125 and two gate bus lines 122. The pixel electrode 128 is connected to the source electrode 126s via a hole in the second insulating film 127.
The capacitive bus line 123 is hold at a constant potential. If the potential of the drain bus line 125 is varied, the potential of the pixel electrode 128 is also varied based on the capacitive coupling due to the stray capacitance. According to the configuration in FIG. 6, since the pixel electrode 128 is connected to the capacitive bus line 123 via auxiliary capacitances, variation in potential of the pixel electrode 128 can be reduced.
In FIG. 6, a color filter 132, a black matrix 133, a common electrode 134, and an alignment film 135 are formed in sequence on an opposing substrate 131 opposing to the TFT substrate 121.
Also, projections 130, 136 that have zig-zag bending patterns to extend in the Y direction are formed on the opposing surfaces of the opposing substrate 131 and the TFT substrate 121 respectively. A bending angle of the bending pattern is roughly 90 degrees.
The projections 130 formed on the TFT substrate 121 side are aligned at an equal interval in the X direction, and their bending points are positioned in the almost center of the gate bus lines 122. The projections 136 formed on the opposing substrate 131 have a pattern substantially similar to the projections 130 formed on the TFT substrate 121, and are formed on the common electrodes 134 such that they are positioned in the almost middle portion between a plurality of projections 130 on the TFT substrate 121.
The projections 130 on the TFT substrate 121 side and the pixel electrodes 128 are covered with the alignment film 129, while the projections 136 on the opposing substrate 131 side are also covered with another alignment film 135. Both the projections 130 on the TFT substrate 121 side and the projections 136 on the opposing substrate 131 side intersect with edges of the pixel electrodes 128 at an angle of 45 degrees respectively.
Also, polarizing plates (not shown) are arranged on the surfaces of the TFT substrate 121 and the opposing substrate 131, which do not put the liquid crystal material between them, respectively. These polarizing plates are arranged such that their polarization axes intersect with linear portions of the projections 130, 136 by 45 degrees to form cross-nicol. That is, the polarization axis of one polarizing plate is parallel with the X direction in FIG. 6 and the polarization axis of the other polarizing plate is parallel with the Y direction in FIG. 6.
The TFT substrate 121 and the opposing substrate 131 are arranged in parallel at a distance mutually, and the liquid crystal material 139 is filled into a space between them. The liquid crystal material 139 having the negative dielectric anisotropy is employed, as described above. The projections 130, 136 are formed of material that has the dielectric constant equivalent to or less than that of the liquid crystal material 139.
Next, the alignment of the liquid crystal molecules L when the intermediate voltage is applied to the pixel electrodes will be explained, by taking as an example the case where the slits are formed in the pixel electrode, hereunder.
FIG.7 is a plan view showing positional relationship among the gate bus lines, the drain bus lines, the capacitive bus lines, and the pixel electrode 128 formed on the TFT substrate on which the slits S are provided on the pixel electrode in place of the projections 130 shown in FIG. 5.
In FIG. 7, the pixel electrode 128a is divided into a plurality of areas by a plurality of slits S passing between upper projections 136a. These areas are conductively connected mutually by connecting portions 128b that are formed to cross the slits S. Two slits S formed in the neighborhood of the center of the pixel electrode 128a are intersected with each other at the edge portion of the pixel electrode 128a. 
Then, when the intermediate voltage is applied to the pixel electrode 128a, the liquid crystal molecules L on the pixel electrode 128a are tilted to the surface of the pixel electrode 128a. The liquid crystal molecule L in FIG. 7 is indicated by a circular cone. A vertex of the circular cone indicates a position of one end of the liquid crystal molecule on the TFT substrate side, and a base of the circular cone indicates a position of the other end of the liquid crystal molecule. Four types of the tilt direction of the liquid crystal molecule L are given based on the principle shown in FIG. 4.
As described above, the MVA mode is the mode in which the liquid crystals having the negative dielectric anisotropy are aligned substantially perpendicularly to the substrate surface. Since the MVA mode can have the high contrast and can improve the visual characteristic without reduction of the switching speed, its display quality is good. In addition, the viewing angle characteristic can be improved much more by using the domain defining means.
FIG. 8 is a sectional view showing another MVA liquid crystal display device in the prior art. First projections 167 are formed on the opposing surface of a glass substrate 151, and second projections 168 are formed on the opposing surface of a glass substrate 186. The first projections 167 and the second projections 168 extend in the direction perpendicular to the sheet of FIG. 8, and are arranged alternately along the lateral direction in FIG. 8. A vertical alignment film 178 is formed on the opposing surfaces of the glass substrates 151, 186 respectively to cover the projections 167, 168.
Liquid crystal material 179 containing liquid crystal molecules 180 is filled between the glass substrate 151 and the glass substrate 186. The liquid crystal molecules 180 have the negative dielectric anisotropy. The dielectric constant of the projections 167, 168 is lower than that of the liquid crystal material 179. Polarizing plates 181, 182 are cross-nicol-arranged on the outside of the glass substrate 151 and the glass substrate 186 respectively. Since the liquid crystal molecules 180 are aligned vertically to the substrate surface when the voltage is not applied, the good dark state can be obtained.
When the voltage is applied between the substrates, equipotential surfaces indicated by a broken line 166 appear. Since the dielectric constant of the projections 167, 168 is smaller than that of the liquid crystal layers, the equipotential surfaces 166 in the neighborhood of the side surfaces of the projections 167, 168 are inclined to come down in the projections. Therefore, the liquid crystal molecules 180a in the neighborhood of the side surfaces of the projections 167, 168 are tilted to become parallel to the equipotential surfaces 166. The peripheral liquid crystal molecules 180a are tilted by the influence of the tilting of the liquid crystal molecules 180a. For this reason, the liquid crystal molecules 180 between the first projections 167 and the second projections 168 are aligned such that their major axis (director) is inclined right-upward in FIG. 8. The liquid crystal molecules 180 positioned on the left side rather than the first projections 167 and the liquid crystal molecules 180 positioned on the right side rather than the second projections 168 are aligned such that their major axis (director) is inclined right-downward in FIG. 8.
In this manner, a plurality of domains in which the tilt directions of the liquid crystal molecules are different are defined in one pixel. The first projections 167 and the second projections 168 define boundaries of the domains. Two type domains can be formed by arranging the first projections 167 and the second projections 168 in parallel with the substrate surface mutually. Four type domains can be formed in total by bending patterns of these projections by 90 degrees. Since plural domains are formed in one pixel, the visual characteristic in the half tone display state can be improved.
The inventors of the present invention point out that the above liquid crystal display device in the prior art has problems described in the following.
The MVA mode liquid crystal display device can achieve the high picture quality, the high reliability, and the high productivity. However, the VA mode has essentially such a nature that it easily accepts the influence of the electric field because of its weak anchoring force in contrast to the horizontally aligned mode such as the TN mode, and thus the MVA mode partakes of such nature of the VA mode.
Accordingly, as shown in FIGS. 9A and 9B, the alignment state of the liquid crystal molecules L around the pixel electrode 128 is changed because of changes of the gate bus line potential Egc and the drain bus line potential (data voltage) Egs in some case. Such phenomenon occurs similarly in the case of the TN mode, nevertheless the phenomenon is ready to occur in the VA mode rather than the TN mode.
Also, as the phenomenon peculiar to the MVA mode, sometimes the projections are charged under various conditions such as the driving state. At this time, the alignment of the liquid crystals at the intersecting portions between the drain bus lines and the gate bus lines is changed by the influence of the charge of the projections.
When the alignment around the pixel is changed, values of the stray capacitances, e.g., a gate-common electrode capacitance Cgc, a gate-source capacitance Cgs, a drain-common electrode capacitance Cdc, etc. are also changed correspondingly. As a result, the potential of the pixel electrode 126s is also changed by the capacitive coupling. Normally the potential variation of the pixel electrode is reduced by the auxiliary capacitance, but such variation cannot be perfectly compensated in some cases. The potential variation of the pixel electrode is easily caused if the auxiliary capacitance is reduced to increase the aperture ratio especially. If the potential of the pixel electrode is varied, the flicker appears on the screen.
It may be considered that the auxiliary capacitance is increased to such extent that the potential variation of the pixel electrode can be eliminated completely. If so, the aperture ratio is reduced correspondingly.
Next, generation of residual images in the MVA liquid crystal display device will be explained hereunder.
The generation of residual images in the liquid crystal display device is caused by the abnormality of the response speed. This is because the domain control direction on the above projections on the electrode and on the above slits is not defined.
Such unstability of the domain control direction is generated due to variation in cell thickness, etc. Hence, the liquid crystal display device in which the residual images are caused is not forwarded as the defective product.
As the result of the examination to check the cause for the long-time remaining residual images, followings become apparent.
In other words, as shown in FIGS. 10A and 10B, in the liquid crystal display device employing the configuration in which a plurality of projections or slits are formed on the electrodes, it can be understood that, if there is a difference between the domain state when the display is changed from the black to the white and the domain state when the display is changed from the half tone to the white, the long-time remaining residual images are generated.
In FIG. 10A, the number of domains on the slits S after the display is changed from the black to the white are six because the domain is divided by boundaries at middle positions (center positions of the slits S) between all the connecting portions 128b of the pixel electrode 128a. Therefore, the liquid crystal molecules L in the neighborhood of the slits S are aligned in the perpendicular direction to the straight portions of the slits S.
In contrast, in FIG. 10B, the number of domains on the slits S after the display is changed in the order of the black, the half tone, and the white are two or four because the domain is divided by boundaries between a part of the connecting portions 128b. Therefore, there exists an area in which the domains are not changed by boundaries between the connecting portions 128b and their middle portions. The liquid crystal molecules L in vicinity of the slits S are aligned obliquely to the straight portions of the slits S in this area.
One of the causes may be considered as follows. That is, since the voltage is not sufficiently applied to the liquid crystal molecules L on the projections 130 or the slits S in the half tone display, the liquid crystal molecules L are aligned almost perpendicularly to the substrate surface, as shown in FIG. 11. Thus, influences of the electric field at the edge of the pixel electrode 128a and the alignment of the display domains being affected by such electric field affect the divided portions of the alignment controlling means as the connecting portions. As a result, the alignment control effect achieved by dividing the alignment controlling means cannot be sufficiently performed. In other words, when the liquid crystal molecules L on the slits S or the projections 130 are aligned perpendicularly in the half tone display, such neighboring liquid crystal molecules L are affected by the electric field at the edges of the pixel electrode 128a and then tilted to the straight portions of the slits S or the projections 130.
Accordingly, when the display is changed from the half tone display to the white display, the domain {circle around (3)} shown in FIG. 10A disappears to connect the domains {circle around (2)} and {circle around (4)}, and then the domain {circle around (5)} disappears to connect the domains {circle around (4)} and {circle around (6)}. As a result, as shown in FIG. 10B, the right-upward directed domains are connected and the left-downward directed domains are disappeared, so that the domains on the slits S after the white display are reduced into two domains {circle around (1)} and {circle around (2)}.
As another one of the causes for generating the residual images, it may be considered that the bending portions of the patterns of the projections 130 or the slits S of the alignment controlling means are arranged at the edges of the pixel electrode 128a. The alignment states of the liquid crystal molecules L at the bending portions are any of three types shown in FIGS. 12A to 12C.
However, the alignment at the bending portions becomes as shown in FIG. 12C since it is affected by the influence of the alignment by the edges of the pixel electrode 128a. As a result, as indicated by a dot-dash line in FIG. 13, the alignment control direction by the edges of the pixel electrode 128a is extended into the pixel. Since this extension affects the alignment of the domains on the slits S in the case of the half tone display, the alignment control effect given by dividing the alignment controlling means cannot sufficiently be brought about.
Also, as shown in FIG. 14A and FIG. 14B, in the TFT substrate, sometimes the area in which a plurality of electrodes are stacked, especially the pixel electrode 128a and the capacitance electrode (capacitive bus line) 123 are punched through the insulating film between them to generate the short-circuit. At this time, in the liquid crystal display device having the structure in which the pixel electrode 128a is divided into a plurality of areas by using the slits S as the alignment controlling means and then these areas are electrically connected by the connecting portions 128b, as indicated by an X mark in FIGS. 14A and 14B, the short-circuited area is disconnected from other areas by cutting off the connecting portions 128b near the TFT 126 in the area of the pixel electrode 128a, that is short-circuited to the capacitive bus line 123, so that the liquid crystal molecules in the pixel can be partially driven.
However, since the area that is short-circuited to the capacitive bus line 123 of the pixel electrode 128a is positioned in the center of the pixel, merely the half area or less of the pixel electrode 128a can be driven, as indicated by a dot-dash line in FIG. 14A, whereby this pixel area acts as the point defect failure to lower yield of the device.
When the voltage is not applied, the liquid crystal molecules in vicinity of the edges of the projections 167, 168 in the MVA liquid crystal display device in the prior art shown in FIG. 8 are aligned almost perpendicularly in the area in which the projections 167, 168 are not formed. However, the liquid crystal molecules in the neighborhood of the edges of the projections 167, 168 are affected by the inclined surfaces of the projections and thus tilted to the substrate surface. Therefore, the double refraction effect appears against the light being transmitted in the thickness direction of the liquid crystal layer. Because of this double refraction effect, the light is transmitted slightly when the display is to be in the dark state, and thus lowering of the contrast is brought out.
The leakage of light in the dark state can be prevented by covering the areas located in the neighborhood of the inclined surfaces of the projections with a light-shielding film. Nevertheless, if such light-shielding film is provided, the light is shielded even in the light state and thus reduction of the transmittance (the aperture ratio) is brought about.
Also, in the MVA liquid crystal display device shown in FIG. 8 in the prior art, the liquid crystal molecules 180 are tilted when the voltage is applied, but the tilt directions of the liquid crystal molecules in the area located far from the projections 167, 168 are not directly decided. That is, the liquid crystal molecules 180a in the neighborhood of the projections 167, 168 are tilted and the tilt is propagated sequentially up to the area far from the projections 167, 168. In this manner, the tilt directions of the liquid crystal molecules 180 in the area far from the projections 167, 168 are indirectly decided. Since distortion of the electric field is small at the time of the half tone display state, the propagation speed of the tilt of the liquid crystal molecules is lowered. Therefore, the response from the dark state to the half tone state is delayed.
Also, the transmission loss of the light is ready to generate in the neighborhood of the projections provided in the MVA liquid crystal display device. Therefore, there is such a tendency that the transmittance (aperture ratio) is reduced in contrast to the TN mode liquid crystal display device. In case the liquid crystal display device is used as the floor type one, the reduction in the transmittance does not become a large issue. Nevertheless, in order to install the liquid crystal display device onto the mobile device, it is desired to enhance the transmittance.
Upon the progress of the lower consumption power of the liquid crystal display device, it is one of important subjects to increase the aperture ratio. In the MVA mode liquid crystal display device, the alignment division (multi-domain) can be accomplished by forming the domain defining projections (so-called banks) on the TFT substrate and the opposing substrate respectively, and thus the good viewing angle characteristic and the good picture quality can be derived. In this case, the aperture ratio is reduced because of the projections in the pixel area.